Light source utilizing diffusive reflective cavity having two oppositely inclined surfaces

ABSTRACT

A light source for backlighting a display comprises a light-emitting device such as an incandescent bulb or LED array disposed within a cavity having diffusely reflecting walls and an aperture. A diffuser and brightness enhancing film are situated at the opening of the aperture between the aperture and the display to be backlit. A color filter may also be employed to whiten the light emerging from the light source.

This application is a division of Ser. No. 08/923,207 filed Sep. 4,1997, which is a division of Ser. No. 08/317,209 filed Oct. 3, 1994 nowU.S. Pat. No. 5,684,354, which is a continuation in part of applicationSer. No. 08/131,659, filed Oct. 5, 1993 now U.S. Pat. No. 5,440,197.

BACKGROUND OF THE INVENTION

The present invention relates to a backlighting apparatus for displays,particularly for small-area liquid-crystal displays (LCDs), such asutilized in virtual reality headsets. LCDs, which allow the display ofalphanumeric, graphic or other information, may comprise a medium whosetransmittance changes in response to the application of an electricalpotential across the medium. The LCDs may be viewed even in an otherwisedark environment by applying illumination uniformly to their rear face.An exemplary backlighting apparatus for an LCD is disclosed in U.S. Pat.No. 4,043,636.

It is desirable for a backlight for small-area LCDs, such as those foundin helmets of virtual reality systems, to have certain attributes.Firstly, it must have an acceptable level of brightness. Secondly, it ispreferable to have a spectral distribution that is as white as possible,particularly if the LCDs display color images. The light source must becompact, and preferably require little maintenance. Lastly and mostimportantly, the lighting system must provide uniform illuminationacross the entire area of the display. This latter need translates intoa requirement that the light emerging from the light source befeatureless and free of distortions such as holes or rings. In practice,the requirement of uniform illumination is difficult to achieve, andprior art devices frequently fail to provide a sufficiently uniformsource of illumination for LCD displays. Additionally, prior art devicesfrequently relied on light guides to direct light to reflectivesurfaces, necessitating complicated geometries and added weight andexpense.

An object of the present invention is therefore to provide a simple,compact, lightweight means for backlighting a display, typically asmall-area LCD display, which provides highly uniform, high-intensityillumination of the entire display panel.

SUMMARY OF THE INVENTION

The light source of the present invention backlights a rear surface of adisplay panel, and includes a housing having diffusely reflectiveinterior surfaces which form a cavity. A device that emits light, forexample, an incandescent light bulb or LED array, is mounted in thecavity with the interior surfaces of the cavity spaced therefrom. Thehousing has an aperture juxtaposed with the rear surface of the displaypanel which opens into the cavity. The ratio of the area of the apertureto the sum of (i) the area of said aperture and (ii) the diffuselyreflective surface area of the cavity is at least 0.05 in a preferredembodiment of the present invention. The ratio of the depth of thecavity to an edge to edge dimension of the aperture is at least 0.1. Theaperture of the embodiment disclosed also has a bisector dimension,defined as the edge-to-edge length of the aperture along a line formedby the intersection of the plane of the aperture and a plane normal tothe plane of the aperture extending through the bulb and bisecting theaperture. The ratio of the depth of the cavity to its bisector dimensionis at least 0.1 in a preferred embodiment of the present invention. Inone embodiment of the present invention, the depth of the cavity is notsubstantially greater than the diameter of the envelope of the lightbulb.

The light source also comprises a diffuser placed across the apertureand positioned to diffuse illumination which passes through the aperturefrom the cavity toward the display panel. A brightness enhancingmaterial for passing illumination within a viewing range is disposedbetween the diffuser and the display panel. In a preferred embodimenthaving orthogonally oriented brightness enhancing films, the viewingrange is 50 degrees. This range is the sum of a pair of angles of 25degrees measured relative to lines normal to the plane of the aperture.

If desired, a color filter may be included between the cavity and theLCD. In the preferred embodiment, the filter is placed between thebrightness enhancing film (BEF) and the rear surface of the display toincrease the color temperature of the light incident on the display.

In one embodiment of the present invention, the light bulb is positionedin a portion of the cavity that is outside of a viewing apertureportion, so that the filament is not visible through the aperture withinthe viewing angle. In another embodiment of the present invention, thelight bulb is located within the viewing aperture portion of the cavitybeneath the aperture. A baffle in front of the lamp reflects lighttowards the bottom of the cavity, and prevents the bulb from directlyemitting illumination through the aperture, thereby preserving theuniformity of the light emerging from the aperture. In yet anotherembodiment, the light is produced by an LED array. The LEDS have colors(e.g. red, blue, and green) and intensities which produce, incombination, light that is white in color.

In all embodiments, the emitted light is diffusely reflected within theinterior surfaces of the cavity, such that the cavity effectivelyfunctions as a lambertian light source. The diffuser gives thetransmitted light a more uniform intensity distribution. The brightnessenhancing film (BEF) concentrates the light emerging from the diffuserby projecting it into a smaller angular viewing range, and therebyenhances the intensity within the viewing angle. Finally, a colorfilter, which is typically blue for incandescent light, may be used tochange the color temperature of the incandescent light from 2800 K-3300K to around 4500 K-5500 K, thereby providing a whiter color.

The invention also encompasses a method of backlighting a display panelcomprising the step of producing illumination from a substantiallylambertian light source having a cavity with internal walls and anaperture. The producing step comprises the step of directing light raysfrom the perimeter of the aperture into the cavity such that the lightexiting the aperture is substantially uniform in intensity and color.The method also includes the steps of using a, diffuser to diffuse lightfrom the substantially lambertian light source using abrightness-enhancing film to concentrate the diffused light into apredetermined angular range without significantly reducing theuniformity of the diffused light, and directing the concentrateddiffused light onto the display panel.

The backlighting apparatus of the present invention producesillumination of a very uniform character, with relatively high intensityand whiteness, in a device that is both simple and compact.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a virtual reality headset incorporatingan LCD backlit by the light source of the present invention.

FIG. 2 is a perspective view of an embodiment of the light source of thepresent invention.

FIG. 3 is a cross-sectional view of the light source of FIG. 2 takenalong the lines 3—3 of FIG. 2.

FIG. 4 is an end-on cross-sectional view of the light source of FIG. 2taken along the lines 4—4 of FIG. 2.

FIG. 5 is a plan view of the light source of FIG. 2 taken along thelines 5—5 of FIG. 3.

FIG. 6 is a graph showing the intensity of light emitted from the lightsource shown in FIGS. 2 as a function of viewing angle.

FIG. 7 is a fragmentary view in cross section, of a brightness enhancingfilm as shown in FIG. 3.

FIG. 8 is a cross sectional view of a second embodiment of the presentinvention.

FIG. 9 is a plan view of the embodiment of FIG. 8.

FIG. 10 is an schematic view of the light source showing the viewingangle.

FIG. 11 is a plan view of an alternate embodiment of the light sourcewhich is identical in all respects to the embodiment of FIG. 2 exceptthat the light source is mounted transversely to the configurationdepicted in FIG. 2.

FIG. 12 is a cross-sectional view of an embodiment utilizing anarrangement of red, blue, and green LEDs which in combination producewhite light.

FIG. 13 is a plan view of the light source of FIG. 12 taken along thelines 13—13 of FIG. 12.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1, one preferred embodiment of the present inventioncomprises a virtual reality headset 5 configured to be worn on the headof a user. The headset 5 has a pair of small-area color display panels26 disposed within an interior cavity 7 of the headset. Each of thepanels 26 comprises an LCD with a front face and a rear face ofidentical size. Each of the front faces of the panels 26 is positionedbehind an eyepiece (not shown) that is directly in front of a respectiveeye of the user so that each eye views the front face of a single one ofthe panels. By way of example, each of the faces of the display panels26 is rectangular, having a length of ½″ and a width of ½″. The headsetis configured to fit against the face of the user sufficiently tightlyso that light from ambient sources cannot enter the cavity 7. Whenproperly mounted on the user's head, the headset will always be insubstantially the same position relative to the user's face, and thedisplay panels 26 will thus also always be in substantially the sameposition with respect to the user's eyes. Accordingly, each time theheadset is worn, the eye of the viewer will have the same viewingposition with respect to the display screens 26.

Each of the panels 26 includes a backlighting apparatus 10, as shown inFIG. 2 and 3. The backlight 10 is comprised of a bulb 12, located withina housing 14 having an aperture 18, covered by a diffuser 20, abrightness enhancing film (BEF) 22, and a color filter 24. The diffuser20, BEF 22, and color filter 24 are placed in physical contact with eachother, forming a planar structure hereinafter referred to as a lightquality enhancing (LQE) apparatus 27. The LCD display 26, which isbacklit by light emerging through the aperture 18 from cavity surfaces16, is juxtaposed with the LQE apparatus 27, so that the display 26 maybe viewed from a position 28. Although the headset 5 has not been shownin FIG. 2 for clarity of illustration, it will be understood that theposition 28 would be within the interior cavity 7 of the headset 5.

In one embodiment of the present invention, the housing 14 has lateralinternal dimensions of 13 mm×33 mm, and the thickness or depth of thehousing is 10 mm, slightly larger than the diameter of a standardflashlight bulb. The aperture 18 is 13 mm square in this embodiment. Theaperture 18 is preferably smaller than 2 inches square to ensure uniformillumination of the cavity walls 16 opposite the aperture 18 by theincandescent bulb 12.

The lamp 12 is preferably an incandescent light bulb such as a commonflashlight lamp powered by a power source (not shown) connected to wires19. The lamp 12 is preferably entirely enclosed in the housing 14 sothat only the wires 19 emerge from the housing 14. The wires 19 passthrough a small passageway in a wall of the housing 14. The passagewayis just large enough to accommodate the wires 19 and is substantiallysmaller than the diameter of the bulb, thereby minimizing light leakageemanating from the annular space, if present, between the wires and thehousing 14.

Light radiating from the lamp 12 propagates within the housing 14, whichforms a cavity comprising diffusely reflecting interior surfaces 16preferably having a reflectivity of at least 88%. The surfaces 16 may becoated with white paint or more exotic materials such as the LabsphereCorporation's Spectraflect paint. Spectraflect paint's reflectivity isconsiderably higher than house paint, roughly 98%, while thereflectivity of house paint is approximately 92%. Additionally, thehousing 14 may be entirely constructed from a diffusely reflectingmaterial, such as TiO₂ pigmented Lexan™ polycarbonate or Spectralon™plastic, thereby avoiding the need to apply a separate coating to theinterior surfaces 16 of the housing 14. The reflectivity of Spectralon™plastic is about 99%.

Light emerging from the lamp 12 typically undergoes several reflectionswithin the cavity formed by the housing 14 before eventually emergingfrom the aperture 18. Because the interior surfaces 16 of the housing 14are diffusely reflecting, the multitude of diffuse reflections cause theinterior surface of the housing 14 opposite the aperture 18 to beuniformly lit and act as a substantially lambertian source, i.e., alight source having the property that the radiance or brightness of theinterior cavity surfaces is constant as a function of viewing angle.

In constructing the light source 10 of the present invention, there area number of factors to consider. One of these is the area of theaperture 18. Another is the combined cavity area, that is the sum of (i)the surface area of the cavity interior 16 and (ii) the area of theaperture 18. For maximally efficient use of the energy emitted from thebulb 12 and uniformity of the emitted light from the aperture 18, it isimportant that the ratio of the area of aperture 18 to the combinedcavity area be relatively high to ensure minimal energy loss within thelight source. Preferably, this ratio is at least 0.05, and in thepreferred embodiment the ratio is close to 0.20. Referring to FIG. 4,the side cavity walls 16 are curved; such an arrangement decreases thecombined cavity area, and thus increases the aforementioned ratio of thearea of the aperture to the combined cavity area. The bottom cavity wallmay also be curved as well to decrease the angle 6 (shown in FIG. 3) asa function of increasing distance from the bulb. The angle δ is theangle formed between a line drawn from the filament of the bulb 12 andthe outward-pointing normal to a plane tangent to the cavity surface ateach point.

Another parameter of importance shown in FIG. 5, is the edge-to-edgedimension 31 of the aperture 18, referred to herein as the bisectordimension. This bisector dimension extends along a line formed by thejuncture of the plane of the aperture and a plane normal to the plane 41of the aperture 18 extending through the bulb 12 and bisecting theaperture. The ratio of the depth of the housing 14 to this bisectordimension affects both the intensity and uniformity of the light visiblethrough the opening of the aperture 18. If this ratio is too large, theaforementioned ratio of the area of the aperture to the combined cavityarea becomes too small. With a small ratio, light from bulb 12 will alsoundergo fewer reflections from the cavity walls 16 and undergo 1/r² falloff, resulting in a less-uniform intensity distribution. In thepreferred embodiments of the present invention, the ratio is more thanapproximately 0.10.

Still another parameter of the present invention is the ratio of thediameter 30 of the envelope of the lamp 12 (typically the width of thebulb) to the depth 32 of the cavity formed by the housing 14. In thepreferred embodiment of the present invention, this ratio issufficiently high to achieve compactness. The housing 14 is preferablyconstructed so that the depth 32 is not substantially greater than thediameter 30 of the bulb. By way of example, the ratio may be 0.60.

While the housing 14 and the aperture 18 basically function as a sourceof uniform light intensity even in the absence of the diffuser 20, inthe preferred embodiment of present invention the diffuser 20 isadvantageously placed in the opening of the aperture 18 to removeresidual nonuniformities such as cosmetic imperfections in the interiorsurfaces 16 of the cavity. The diffuser is comprised of translucentmaterial, typically a readily available thin plastic surface or volumediffuser. Both of these materials are preferred because they arecharacterized by very low absorption, thus minimizing energy losses.

To avoid wasting optical energy, it is preferable to match the numericalaperture of the backlight with the numerical aperture of the optics(e.g., the eyepiece of the headset 5) that receives the light from thebacklight. Because the cavity acts as a substantially lambertian source,it is necessary to decrease the numerical aperture of the backlight, andthereby concentrate the light emanating from the aperture. Inparticular, the backlight 10 employs the BEF 22 placed between thediffuser 20 and the display 26 to concentrate the illumination, andthereby increases the brightness.

It is helpful for present purposes to define an angle whichcharacterizes the performance of BEF 22. This angle, θ_(t) (shown at 39in FIG. 10), is the semi-angle from a normal 36 to the plane of theaperture 18. More specifically, the BEF 22 transmits light rays withinθ_(t). Except for weak side lobes, no light will be transmitted by theBEF beyond the angle θ_(t).

A second angle, the viewing angle θ_(v) shown at 41, subtends twice theangle θ_(t) of the BEF 22. Consequently, θ_(v)=2θ_(t).

It will be understood that concentration of the illumination by the BEF22 (within the angle θ_(t)), shown in FIG. 10, is symmetrical only inthe sense that concentration occurs within a plane coincident with theplane of the paper in FIG. 10. The BEF 22 does not provide concentrationwithin the orthogonal plane. In some applications of the invention, itis preferable to concentrate the illumination in both of the orthogonalplanes. This may be accomplished by including. a second BEF orientedorthogonally to the BEF shown in FIG. 10. Such an arrangement wouldprovide a boundary line 37 at each of the four edges of the aperture 18and thereby concentrate the illumination so that substantially noradiation beyond the angle θ_(t) is transmitted from the aperture 18.

Referring to FIG. 7, the BEF of the preferred embodiment is acommercially available thin film having linear pyramidal structures 38.In principle, the structures 38 transmit only those rays from the cavitythat satisfy the incidence angle criteria for transmission into thetransmission region bounded by the boundary lines 37. All other rayswill be reflected back into the cavity, where they are diffuselyreflected again by the cavity walls. In effect, the reflected rays are“recycled” until they are incident on the BEF at an angle which permitsthem to pass through the BEF into the transmission region.

The fraction f of light retroreflected by the pyramidal structures 38 ofthe BEF 22 satisfies the relationship 1-f≈sin θ_(t). Thebrightness-enhancing effect results from the fact that many of theretroreflected rays are themselves diffusely reflected and eventuallyare transmitted by the BEF 22. Because the BEF 22 is designed so thatθ_(t)<90 degrees, it concentrates light within the display range,thereby increasing the intensity of light seen within this range usingpyramidal structures, a 40% gain over an unenhanced lambertian sourcehas been observed. If a film having orthogonally oriented rows is used,a gain of as much as 80% may be possible. The use of orthogonallyoriented films produces the enhancing effect in two orthogonal planesrather than only along the axis perpendicular to the pyramidalstructures, as is the case when only one set of pyramidal structures 38is used. In the preferred embodiment, the structure of the film issufficiently fine that it is imperceptible to the viewer of the display26 and the light intensity resulting therefrom is as uniform aspossible. Referring to FIG. 7, the BEF 22 may be a film having an apexangle α (typically about 100°). Such film is available from 3M. As analternative to two orthogonally oriented lenticular films of the typeshown in FIG. 7, an array of two-dimensional micro lenses may beutilized.

The concentrating effect of the BEF 22 is depicted in FIG. 6, which is agraph of the brightness of light observed as a function of viewing angleψ (here, viewing angle ψ is defined as the angle the eye of the observermakes with the plane of the aperture 18). Graph 40 illustrates theintensity as a function of viewing angle without the BEF 22, while graph42 shows a distribution achieved with the BEF 22. It can be seen thatthe intensity achieved within the window θ_(v)=2θ_(t) degree wide isgreater than that achieved by use of the diffuser alone. It is alsoimportant to note that the BEF is placed between the diffuser and thedisplay, since the latter has a spreading effect on the angulardistribution of light, while the former concentrates it.

FIG. 6 also illustrates an advantageous feature of the presentinvention, namely the uniformity of the distribution of illuminationthroughout the viewing range 60. It can be seen that the graph 42 of thelight intensity emerging from the BEF 22 as a function of viewing angleψ is highly uniform throughout the entire viewing angle θ.

Light emerging from the BEF 22 passes through the color filter 24 in apreferred embodiment of the present invention. The color temperature ofthe flashlight bulb that comprises the lamp 12 is only about 2800 K.However, color LCD displays require a higher color temperature toachieve ideal color purity. Consequently, when an incandescent source isemployed as the lamp 12, a filter may be used to shift the colordistribution as desired. The filter 24 of the preferred embodiment is ablue absorbing filter that shifts the color temperature to between about4500 K and 5500 K. Light passing through the filter 24 continues throughthe LCD 26 to the eye of the viewer located at the position 28. The gapsbetween elements 20, 22, and 24 in FIG. 1 are depicted only for clarity;the thicknesses of and separation between each of the various elementsare minimized for the sake of compactness.

In the embodiment of the backlight illustrated in FIGS. 2-3, the lamp 12is situated in the housing 14 so that it is outside of a viewingaperture portion 17. As used herein, the term “viewing aperture portion”refers to the portion of the cavity that lies directly beneath theaperture 18. Placement of the lamp 12 outside the viewing apertureportion 17 prevents most of the light rays emanating from the lamp 12from reaching the aperture 18 without first being reflected off asurface of the cavity. Since only glancing rays from the bulb 12directly impinge on the LQE 27 and the diffuser 20 scatters these rays,this arrangement allows the intensity distribution of light emergingfrom the aperture 18 to be relatively uniform. An alternate embodimentof the backlighting system allowing even more compact construction isillustrated in FIGS. 8-9, in which corresponding numbers denote likeparts. The lamp 12 is placed in a different portion of the housing 14.In this embodiment, the lamp 12 is placed directly within the viewingaperture portion 17. The lamp 12 is shielded from the aperture 18 by anopaque baffle 56. The baffle 56 has two diffusely reflecting outersurfaces which are coated with one of the diffusely reflecting materialsdescribed above. The diffusely reflecting surfaces of the baffle 56prevent the lamp 12 from directly illuminating the aperture 18, whilereflecting light incident thereon, such as any rays reflected backtoward the aperture from the diffuser and the BEF, thus preserving theuniformity of the light distribution. Since the lamp 12 is directlybeneath the aperture, as opposed to being set back in the housing 14outside viewing aperture 17, the housing 14 can be more laterallycompact than that of the embodiment of FIG. 2. Additionally, theembodiment of FIG. 8 allows a higher ratio of the aperture surface areato the combined cavity area, thus allowing even greater efficiency inthe use of energy emanating from the bulb 12. Aside from this placementof the lamp, all details of this embodiment, such as the coating of theinterior surfaces of the housing 14 and the placement of the diffuser,BEF and filter, are identical to those of FIG. 2.

A further embodiment, illustrated in FIG. 11, is identical to theembodiment shown in FIGS. 1-5, except for the orientation of the bulb12. Accordingly, like numbers designate like parts. In the embodiment ofFIG. 11, the bulb is oriented so that a line extending along itslongitudinal axis is parallel to, but spaced from, the aperture (asopposed to the embodiment shown in FIG. 5, where a line extending alongthe longitudinal axis of the bulb passes beneath the aperture). Thus,the bulb in FIG. 11 is rotated 90° relative to the bulb in FIG. 5.

Yet another embodiment of a lambertian light source is shown in FIGS. 12and 13. In this embodiment, red, blue, and green light from lightemitting diodes (LED) is mixed together in a manner well known in theart to produce white light. For clarity of illustration, partscorresponding to like parts of prior embodiments will be designatedusing like numbers that are primed. As illustrated, a housing 14′comprises a diffusively reflecting cavity having interior cavity walls16′ and an aperture 18′ which opens into the cavity. The dimensions ofthe aperture 18′ are 16.1 mm by 14.1 mm for the particular arrangementshown, which uses 2 red LEDs 12′(r), 4 blue LEDs 12′ (b), and 18 green:LEDs 12′(g). As shown in FIG. 13, the LEDs are mounted around theperiphery of the aperture 18′ within a channel 70 that extends aroundthe entire perimeter of the aperture 18′. The channel 70 is formed by asmall baffle 72 that extends from the edge of the aperture 18 a shortdistance into the cavity and along the entire perimeter of the aperture18′. Preferably, the distance by which the baffle 72 extends into thecavity is no greater than is necessary to prevent the LEDs from beingviewed through the aperture 18′. In any event, the baffle 72 is spacedfrom the cavity walls 16 by a sufficient distance to permit light fromthe LED's to diffusively reflect into the portion of the cavity beneaththe aperture 18′. In preferred embodiments, the depth of the cavity is5-10 mm. As is typical of light-emitting diodes, the LEDs 12′ comprisetiny cubes of solid-state material that emit light. In the embodimentshown, the solid-state material is not encased in a housing, and nodirectional reflectors are used such that the emission is allowed topropagate multidirectionally from plural faces of the solid-state cubes.Such multifaceted emission enhances the uniformity of the intensity oflight exiting the aperture 18′.

The LEDs 12′ are positioned so that for each color (red, blue, green)the output from the aperture 18′ is substantially uniform with respectto intensity. In the preferred embodiment, the LEDs 12′ are positionedsymmetrically, with an equal number of diodes 12′ of like color onopposite edges of the rectangular aperture 18′. Thus, as viewed fromFIG. 13, the top edge of the aperture 18′ has nine green diodes 12′(g)and one red diode 12′(r), while the bottom edge of the aperture also hasnine green diodes 12′(g) and one red diode 12′(r). Similarly, the leftedge of the aperture has two blue diodes 12′(b), while the right edgealso has two blue diodes 12′(b). In addition to symmetry with respect toopposite edges of the aperture 18′, the diodes preferably havesubstantial symmetry with respect to sides of the same edge of theaperture 18′. Thus, for example, the single red diode 12′(r) at the topedge in FIG. 13 is placed substantially in the center of that top edgewith five green diodes 12′(g) on the left side and four green diodes12′(g) on the right side. The bottom edge embodies the same symmetryexcept that the four green diodes are on the left side of the red diodeand the five green diodes are on the right side. In regard to the leftaperture edge (as seen in FIG. 13), each of the two blue diodes 12′(b)is positioned so that it is the same distance from an end of the leftaperture edge as it is from the other blue diode. The blue diodes 12′(b)at the right aperture edge have this same symmetry with respect to theright aperture edge.

As mentioned above, the combination of LEDs 12′ is selected to providewhite light. Thus, while a diffuser 20′ and BEF 22′ are included, asshown in FIG. 12, no color filter is necessary because the combinationof red, blue and green colors produces light of sufficient whiteness.However, in some cases it may be necessary to underdrive some of theLEDs in order to obtain the desired color balance and desired whiteness.If so, it is preferable that all LED's of the same color be underdrivenby the same amount so as to preserve color uniformity at the aperture18′. Because the illumination produced by the symmetrical arrangement ofLEDs and the diffusively reflecting cavity yields a substantiallyuniform intensity output at the aperture 18′ for each color, the lightsource produces a high-quality color image.

The present invention thus comprises a highly uniform, efficient, andcompact light source for demanding applications such as small color LCDdisplays in virtual reality systems. However, it also has application inother small-area backlighting systems as well, such as in digitalwatches or automotive gauges. It is understood that the presentdisclosure of the preferred embodiment may be changed in the combinationand arrangement of parts without departing from the spirit and scope ofthe invention hereinafter claimed.

What is claimed is:
 1. A method of lighting, comprising: providing anoptical cavity formed by a member having a bottom portion and peripheralportions surrounding said bottom portion, said peripheral portionscomprising first and second end portions and first and second sideportions, an upper portion of said optical cavity having an aperture foroutputting light from the cavity, producing illumination from at leastone location disposed outwardly from edges of said aperture such thatthe source of the illumination is not visible through the aperture;providing a substantially planar optical element comprising a diffuser;reflecting light upwardly off each of two diffusely reflecting surfaceportions that are oppositely inclined relative to said planar opticalelement and disposed on opposite sides of said cavity; passing lightfrom the cavity to said optical element and through said diffuser; andusing a series of repeated structures on the planar optical element toangularly restrict light output from the aperture.
 2. The method ofclaim 1, wherein said optical element comprises a diffuser and abrightness enhancing film, said method additionally comprising: usingthe diffuser to diffuse illumination passing from the cavity through theaperture; and using the brightness enhancing film to concentrate thediffused light within an angle.
 3. The method of claim 2, comprisingjuxtaposing a display panel with said aperture to backlight said displaypanel, and positioning said diffuser and said brightness enhancing filmbetween said display panel and said aperture.
 4. The method of claim 2,comprising positioning said diffuser between said brightness enhancingfilm and said aperture.
 5. The method of claim 1, further comprisingusing a color filter to change the color temperature of saidconcentrated, diffused light.
 6. The method of claim 1, wherein therepeated structures comprise pyramidal structures.
 7. The method ofclaim 1, wherein said two diffusely reflecting surface portions onopposite sides of said cavity are curved.
 8. The method of claim 1,wherein said producing illumination comprises activating at least onelight emitting diode mounted adjacent said first end portion.
 9. Themethod of claim 1, wherein said reflecting comprises reflecting lightoff two curved diffusely reflecting surface portions that are oppositelyinclined relative to said planar optical element and disposed onopposite sides of said cavity.
 10. The method of claim 9, wherein saidproducing illumination comprises activating at least one light emittingdiode mounted adjacent said first end portion.
 11. The method of claim1, wherein said two diffusely reflecting surface portions on oppositesides of said cavity are disposed substantially in facing relationshipwith respect to each other.
 12. The method of claim 1, wherein saidreflecting comprises reflecting light off two white diffusely reflectingsurface portions that are oppositely inclined relative to said planaroptical element and disposed on opposite sides of said cavity.
 13. Themethod of claim 1, wherein said step of providing an optical cavitycomprises providing a fluid-filled optical cavity.
 14. The method ofclaim 13, wherein said step of providing an optical cavity comprisesproviding an air-filled optical cavity.
 15. An apparatus for lighting,comprising: an optical cavity formed by a member having a bottom portionand peripheral portions surrounding said bottom portion, said peripheralportions comprising first and second end portions and first and secondside portions, said optical cavity having an aperture for outputtinglight from the cavity and a diffusely reflecting cavity surfaceextending between at least two of said peripheral portions, saiddiffusely reflecting cavity surface having a pair of surface portions onopposite sides of said cavity, said surface portions each oriented toreflect light upwardly towards said aperture; one or more light sourcesmounted to illuminate said optical cavity such that (i) substantiallyall of the cavity illumination emanates from at least one locationdisposed outwardly from edges of said aperture and (ii) said one or moresources are not visible through the aperture; a substantially planaroptical element extending across the aperture, said planar opticalelement including a diffuser and having a series of repeated structureswhich angularly restrict light output from the aperture, wherein saidpair of surface portions on opposite sides of said diffusely reflectingcavity surface are oppositely inclined relative to said planar opticalelement.
 16. The apparatus of claim 15, wherein said optical elementcomprises a diffuser and a brightness enhancing film, said diffuserdiffusing illumination passing from the cavity through the apertures,and the brightness enhancing film concentrating the diffused lightwithin an angle.
 17. The apparatus of claim 16, wherein said diffuser ispositioned between said brightness enhancing film and said aperture. 18.The apparatus of claim 15, wherein said pair of surface portions onopposite sides of said cavity are curved.
 19. The apparatus of claim 15,further comprising at least one light emitting diode mounted adjacentsaid first end portion.
 20. The apparatus of claim 19, wherein said pairof surface portions on opposite sides of said cavity are curved.
 21. Theapparatus of claim 15, wherein said one or more light source compriseslight emitting diodes mounted adjacent said first and second side endportions.
 22. The apparatus of claim 15, wherein said pair of surfaceportions on opposite sides of said cavity are disposed substantially infacing relationship with respect to each other.
 23. The apparatus ofclaim 15, wherein said diffusely reflecting cavity surface comprisesdiffusely reflecting white material.
 24. The apparatus of claim 15,wherein said optical cavity is fluid filled.
 25. The apparatus of claim24, wherein said fluid is air.